Hairs show cycles of growth and rest. Each individual has unique cycle. Growth phase of hair is known as anagen and it lasts two to three years during which hair grows about 1 centimeter a month. The resting phase is called telogen which lasts for four months. After the resting phase the hairs falls and a new one begins to grow in its place.

 

Once hairs are shed, the growth stage begins again. Shedding 50 to 100 hairs a day is normal. But with about 100,000 hairs in the scalp, there shouldn't be noticeable thinning of the scalp hair. Gradual thinning is a normal as age advances. However, hair loss leads to baldness when the rate of shedding is m ore than the rate of regrowth, when new hairs are than the hair which falls out or if hairs come out in patches. 

Causes of hair loss 

• Pattern baldness or androgenetic alopecia - Two types of patterns exists called as male and female pattern of baldness respectively. In both male and female pattern baldness, the time of growth phase shortens, and the hairs are not as thick and strong.  Hair roots become more superficial and fall out more easily. Heredity is also a cause. A family history of androgenetic alopecia increases the risk of hair loss.
• Cicatricial or Scarring alopecia - This is a type of permanent hair loss and it occurs due to scarring of hair follicles secondary to inflammation thus preventing new hair growth. This condition can be seen in combination with several other skin conditions, including lupus erythematosus or lichen planus. Exact cause of inflammation is not known.
• Alopecia Areata - This is type of alopecia having autoimmune pathology but the exact cause is unknown. Patients are generally good in health. Some persons may have other autoimmune disorders in conjunction like thyroid disease. It is believed that some persons have genetic predisposition to develop alopecia areata and any trigger like a viral infection or some Enviornmental conditions may lead to the condition. A family history of alopecia areata increases the risk of developing the disease. Hair may grow back in such condition but will take a very long time in regrowth.
• Telogen effluvium - This is a type of hair loss is which usually occurs due to a change in your normal hair cycle. It occurs when there is some emotional or some physical trigger or setback which leads hair roots into the resting state. The affected hairs from these hair roots fall. After few months, the hair follicles become active again and lead to the growth of new hair. Telogen effluvium may result from emotional distress, like death of a close relative, or after a physiological stress, like high fever, sudden weight loss, nutritional deficiencies, surgery and metabolic disturbances. Hair may grow back once the condition that caused it corrects but it usually take many months.
• Traction alopecia - Hairstyling or hairstyles leads to pulling of hairs too tightly can cause traction alopecia. If the pulling is stopped before inflammation sets in which leads to scarring of scalp and permanent damage to the root, hair may grow back.
• Inadequate nutrition - Poor protein or iron in diet or poor nourishment in other ways can lead to hair loss. Fad diets, crash diets and few disorders like eating disorders, can lead to poor nutrition and in turn hair loss.
• Medications - Few drugs used to in medical conditions like gout, arthritis, depression, heart diseases and high blood pressure may cause hair loss in some persons. Birth control pills can result in hair loss in some women.
• Disease - Diabetes and lupus can lead to hair loss.
• Medical treatments - Chemotherapy and radiation therapy may lead to alopecia. Under these treatments hairs in anagen phase are affected. After the treatment hair growth will resume.
• Hormonal changes - Hormonal imbalances can lead to temporary hair loss. Following factors contribute in it, pregnancy, having a baby, discontinuing birth control pills, beginning of menopause, or an overactive or underactive thyroid gland. The hair loss may occur after three months following a hormonal change, and it'll take 3-4 months for new hair to grow back. During pregnancy, females will have thicker and more lustrous hair. Females generally lose more hair than normal about three months after delivery. If there is overproduction of testosterone, it may lead to thinning of hair over the crown of the scalp. Once hormonal imbalance corrects may lead to hair growth.
• Hair treatments - Chemicals used for dying, tinting, bleaching, straightening or permanent waves will cause hair damage and ultimately fall of hair if used incorrectly. Over styling and excessive brushing also can lead hair to fall.
• Scalp infection - Infections like ringworm invade the hair and skin of scalp which leads to hair loss. Once the infection is treated, hair may grow back. Ringworm is a fungal infection and is treated with a topical or oral antifungal medication.
• Trichotillomania - Trichotillomania is a mental disease in which patients will have an irresistible urge to pull out their hair, whether it's from their scalp, their eyebrows or other areas of their body. Hair pulling from the scalp results in patchy bald spots on the head, which they may go to great lengths to disguise. Causes of trichotillomania are still being researched, and no specific cause has yet been found.